Plastic films coated with a vinylidene chloride copolymer latex have a characteristic of excellent barrier properties, and are used mainly for food packaging materials and medicament packaging materials in order to block oxygen and water vapor and to store the packaged food and medicaments without being moistened or oxidized for a long period. Of various films, some have excellent barrier properties against either oxygen or water vapor, but there are few films that can block both oxygen and water vapor at a high level except films employing a vinylidene chloride copolymer. Thus, in general, packaging materials coated with a vinylidene chloride copolymer latex are widely used.
Particularly, in blister pack applications represented by medicine packaging, vinylidene chloride copolymer latexes are universally used (for example, see Patent Literatures 1 to 3.)
In recent years, the high level of barrier properties has been required for films for blister packs. Accordingly, performance requirements for the barrier properties of vinylidene chloride copolymer latexes have been enhanced. The barrier properties of coated films generally depend on the thickness of a coating of a vinylidene chloride copolymer latex. The larger the thickness of the coating is, the higher the barrier properties that can be exerted is. However, when a large amount of the vinylidene chloride copolymer latex is coated at a time in order to increase the thickness of the coating of the latex, there occur problems of causing cracks and crinkles in the coating during the drying process. If such a poor appearance takes place in the coating, appearances as commercial products are impaired, and moreover, the film will not be able to exert the expected barrier properties.
Thus, for the purpose of improving the barrier properties per unit thickness of a vinylidene chloride copolymer coating, the types and ratio of the constituent units derived from other monomer(s) constituting the copolymer with vinylidene chloride i.e., the composition of the copolymer, also can be changed for the use. However, when the composition of the copolymer is changed, the problem of a poor discoloration resistance to light and heat easily arises, easily resulting in the problem of deterioration in the appearances as commercial products.
Also, one of the performance requirements for films for blister packs includes impact resistance. Films for blister packs have to exert flexibility against impact forces from the outside (an impact resistance) to thereby maintain the structure of the coating and the barrier properties. Films coated with a vinylidene chloride copolymer latex have to exert an impact resistance immediately after being coated, and to sustain the impact resistance for a long period. For example, in the case where the impact resistance is insufficient in the early stage of coating, cracks and defects may be caused by an impact force applied during slitting or shaping of the film. These cracks and defects may lead to a defective condition where the barrier properties are impaired. Also, in the case where films are stored for a long period, crystallization, which is characteristic of vinylidene chloride copolymers, progresses, and the coating easily becomes brittle. Therefore, due to the insufficient impact resistance, cracks and defects arise, and the problem that the barrier properties are impaired may take place.
Generally, in order to enhance the barrier properties, the proportion of the constituent units derived from vinylidene chloride in a vinylidene chloride copolymer is adjusted to thereby adjust the barrier properties. In the case where the barrier properties are enhanced only by adjusting the proportion, the coating becomes brittle and tends to lack an impact resistance because the crystallinity of the vinylidene chloride copolymer becomes increased. Thus, if this coating is used as a film for blister packs, cracks and defects are easily generated, and the expected barrier properties cannot be exerted. Also, in the case where a wax composition is added to a vinylidene chloride copolymer latex, the coating tends to rapidly become brittle after the coating and to have a drastically reduced impact resistance. Therefore, if this is used as it is as a film for blister packs, defects take place in the film, and thus, the expected barrier properties cannot be exerted.